Low delay decoding

ABSTRACT

A method and apparatus for decoding a frame of interleaved information bits in a communications system, where the decoding of the frame of interleaved information bits may begin before all of the bits in the frame are received at a decoding site. The frame of interleaved information bits has a frame start time and a frame end time. The frame also includes a first fractional segment that has a start time that is the same as the frame start time and an end time that is before the frame end time. Prior to transmission of the frame of interleaved information bits to a decoding site, all bits in the frame are encoded at a code rate R to provide encoded bits, and the encoded bits are positioned in the interleaved frame in accordance with an interleaving pattern that stores bits having a code rate R1 within a first fractional segment of the interleaved frame. In one embodiment, R1=R/a 1 , and a 1  corresponds to the first fractional segment of the frame. At the decoding site, interleaved information bits from the first fractional segment of the frame are initially received, and an attempt is made to decode the frame using information bits from only the first fractional segment of the frame. If this initial decoding attempt is unsuccessful, the system attempts to decode the frame again using information bits from both the first fractional segment of the frame and further information bits located between the end time of the first fractional segment and the frame end time.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to the field of communications systems and, inparticular, to transmission of message signals in a communicationssystem.

II. Prior Art

It is well known that it is desirable to reduce the receive and decodedelay of frames of data transmitted from a transmitter to a receiver ina communications system. For example, it is desirable to reduce framedelay when information is transmitted through a broadcast channel in acellular system. Among other things, a reduced delay will allow mobileunits to access the cellular system in a faster manner.

Some delay can be tolerated when voice telephones access a cellularsystem in order to initiate a call. In the case of cell handoff, if theamount of delay is relatively small then it is unnoticeable to the user.However, if the delay becomes too large, it can result in a call beingdropped by the system.

High data rate terminals are an example where receive and decode delayis critical. These terminals frequently go on and off line and theaccess delay can slow applications and result in congestion at the datasource. Additionally, in soft handoff even a short delay can producesignificant congestion or loss of data.

It is known in the prior art to reduce frame size in order to reducethis delay problem. However, this solution reduces the coding gain orthe amount of data that can be transmitted as well as interleaving gain.In order to partially compensate for the loss incurred using thismethod, the power of the channel can be increased or other methods suchas the R-RAKE technique can be used. Additionally increasing the powerof the channel will decrease the total capacity of the system. Moreover,the R-RAKE compensation technique requires two or more frames in a rowto be the same, and the receiver must be able to know or detect thiscondition. It can be difficult or troublesome for a system to make thisdetermination.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for decodinga frame of interleaved information bits in a communications system. Thepresent invention permits decoding of the frame of interleavedinformation bits before all of the bits in the frame are received at adecoding site. In the present invention, the frame of interleavedinformation bits has a frame start time and a frame end time. The framealso includes a first fractional segment that has a start time that isthe same as the frame start time and an end time that is before theframe end time. Prior to transmission of the frame of interleavedinformation bits to a decoding site, all bits in the frame are encodedat a code rate R to provide encoded bits, and the encoded bits arepositioned in the interleaved frame in accordance with an interleavingpattern that stores bits having a code rate R1 within the firstfractional segment of the interleaved frame. In a preferred embodiment,R1=R/a₁, and a₁ corresponds to the duration of the first fractionalsegment of the frame. At the decoding site, interleaved information bitsfrom the first fractional segment of the frame are initially received,and an attempt is made to decode the frame using information bits fromonly the first fractional segment of the frame. If this initial decodingattempt is unsuccessful, the system attempts to decode the frame againusing information bits from both the first fractional segment of theframe and further information bits located between the end time of thefirst fractional segment and the frame end time.

In accordance with a further aspect, the interleaving pattern describedabove also optionally stores bits having a code rate R2 at a secondfractional segment of the interleaved frame, wherein the starting timeof the interleaved frame and starting time of the second fractionalsegment coincide with each other, and the second fractional segment hasan ending time that is after an ending time of the first fractionalsegment. The interleaving pattern also optionally stores bits having acode rate R3 at a third fractional segment of the interleaved frame,wherein the starting time of the interleaved frame and starting time ofthe third fractional segment coincide with each other, and the thirdfractional segment has an ending time that is after an ending time ofthe second fractional segment. In a preferred embodiment, a₂ correspondsto a length of the second fractional segment of the frame, a₃corresponds to a length of the third fractional segment of the frame,and the interleaving pattern used is chosen such that R2=R/a₂ andR3=R/a₃. Additionally, the bits in the first fractional segment areincluded in the second fractional segment; and the bits in the first andsecond fractional segments are included in the third fractional segment.

In accordance with a still further aspect, when the interleaved frame isformed as described in the paragraph above, if the initial decodingattempt (using only the first fractional segment) is unsuccessful, thesystem attempts to decode the frame a second time using information bitsfrom only the second fractional segment of the frame. If this seconddecoding attempt fails, the system then attempts to decode the frame athird time using information bits from only the third fractional segmentof the frame. Finally, if this third decoding attempt fails, the systemattempts to decode the frame again using all information bits from theframe. In accordance with a particularly preferred aspect, the first,second and third decoding attempts described above may be performedbefore all of the frame has been received by the receiver at thedecoding site.

Many values of a₁, a₂ and a₃ may be used for implementing the presentinvention. In one embodiment described below, a₁, a₂ and a₃ correspondto the values ¼, ½, and ¾, respectively. It will be understood by thoseskilled in the art that other values for these parameters mayalternatively be used for implementing the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify corresponding elements throughout and wherein:

FIG. 1 shows a transmission and reception chain within a communicationssystem;

FIG. 2 shows the method for performing early decoding of a receivedinformation frame in accordance with the method of the presentinvention;

FIGS. 3A, 3B show an exemplary interleaving pattern used forinterleaving frames of encoded bits in accordance with the presentinvention; and

FIG. 4 shows a further method for forming an interleaved frame of codedbits for use in a communications system such as the system of FIG. 1, inaccordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown transmission chain 10 forencoding, transmitting, and decoding data in a communications channel.Information bits within transmission chain 10 are received by framingand encoding block 14, which encodes the bits at a code rate R. Asdescribed below, the rate R is directly related to the amount ofredundancy introduced into the information stream as information istransformed into coded bits by block 14. After the framing and encodingis performed in block 14, coded information bits from block 14 areapplied to channel interleaver block 18 which outputs a frame ofinterleaved bits. As discussed more fully below, interleaver block 18interleaves the coded bits using an interleaving pattern thatfacilitates early decoding of the frame at a decoding site. Moreparticularly, the interleaving pattern is chosen such that theinformation stored in the frame may be successfully decoded using onlybits in a first fractional portion of the interleaved frame. Theinterleaved frames output from block 18 are received, modulated andtransmitted by modulation block 22. The modulated information istransmitted by way of transmission channel 26 to a receiving/decodingsite.

Within a receiver of the communication system, the transmittedinformation is received and demodulated as shown in block 28. Thedemodulated information is deinterleaved in block 32. Decoding is thenperformed in block 36 to provide decoded information bits. As explainedmore fully below, decoding block 36 functions by first attempting todecode the frame using only those bits in a first fractional portion ofthe frame. This first decoding attempt is preferably performed beforeall the bits in a frame are received by decoding block 36. If this firstdecoding attempt fails, decoding block 36 attempts again to decode theframe using additional bits from the received frame.

The performance of the decoding process of transmission chain 10 isrelated to the average energy per coded bit to interference ratioE_(s)/N₀ of the signal received by demodulation unit 28. The performanceof the decoding process is also related to the performance of the codeused in block 14 for coding the information bits. One characteristic ofthe code that is important for its performance is its rate R. The rate Ris directly related to the amount of redundancy introduced into theinformation stream when the information is transformed into coded bitswithin block 14. For example, a code of rate R=¼ produces four codedbits for each uncoded information bit. A lower code rate R produces amore efficient code. A more efficient code requires a lower E_(s)/N₀ forsuccessful decoding of a frame.

In an example, the average signal to noise interference E_(s)/N₀ isconstant throughout a frame. A code used within framing and encodingblock 14 has a rate R. In the prior art, it is typically necessary towait until an entire frame is received before deinterleaving anddecoding it. However, it is easy to perform the deinterleaving as thesymbols are received by the receiver. Thus, there is a delay between thetime that deinterleaved bits can be available and the time when thedecoding can be performed. In order to reduce the delay in accordancewith the method of the present invention, decoding of the frame can beattempted before the reception of the frame is complete.

If channel interleaver 18 of transmission chain 10 is designed as setforth below, the decoding of the frame before the reception is completecan be performed by using only those coded bits in a first fractionalportion of the frame. For example, if the code rate R of the encodingprocess is ¼, the decoding can begin when only half of the frame isreceived. If channel interleaver 18 is appropriately chosen as describedbelow, the coded bits received in the first fractional portion of theframe appear as information bits encoded with R=¼*2=½ rate code.E_(s)/N₀ for the received bits is the same as if the entire frame hadbeen received. If the code is well designed, the performance loss istherefore only the performance loss due to converting the originalinformation bits to a higher rate code, plus half the power (3 dB)because only half the frame is received.

Referring now to FIG. 2, there is shown a graphical representation ofearly decoding method 40 of the present invention. Early decodingprocess 40 is preferably implemented on decoding block 36. In accordancewith early decoding process 40, it is possible to begin decoding a framebefore is entirely received and wait until additional bits are received.The decoding process can be attempted again once more bits have beenreceived if the previous decoding attempt failed. The repeating of thedecoding process can continue until either a successful decoding of theframe or reception of the entire frame.

In early decoding method 40, a first attempt at decoding the receivedframe is performed at time 44 using only those bits in the firstfractional portion of the frame (i.e., the first (a₁*100) % of theframe.) Assuming that the received frame was previously coded andinterleaved as described below, then at time 44 the code rate of thereceived bits is R/a₁, where a₁ represents the time between thebeginning of the frame and time 44 divided by time 52 (where time 52 isthe total duration of the frame.) If the decoding performed at time 44is not successful, another decoding of the frame can be attempted attime 48 using only those bits in the second fractional portion of theframe (i.e., the first (a₂*100) % of the frame.) Assuming again that thereceived frame was previously coded and interleaved as described belowand that the time between the beginning of the frame and time 48 is a₂,the rate of the decoded bits provided at time 48 is R/a₂, where a₂represents the time between the beginning of the frame and time 48divided by time 52. If the decoding performed at time 48 is notsuccessful, the process can then be repeated again using a thirdfractional portion of the received frame (i.e., the first (a₃*100) % ofthe frame.) Assuming that the received frame was previously coded andinterleaved as described below, then at the time of this third decodingthe code rate of the received bits is R/a₃, where a₃ represents the timebetween the beginning of the frame and the end of the third fractionalportion divided by time 52. Finally, if this third decoding attempt isunsuccessful, the frame may be decoded in a fourth attempt using all ofthe bits in the received frame. In the preferred embodiment, the firstthree decoding attempts described above may be performed before all thebits in the frame are received by decoding block 36. More particularly,the first decoding attempt may be performed as soon as the bits in thefirst fractional portion of the frame are received by decoding block 36,the second decoding attempt may be performed as soon as the bits in thesecond fractional portion of the frame are received by decoding block36, and the first decoding attempt may be performed as soon as the bitsin the third fractional portion of the frame are received by decodingblock 36.

Referring now to FIGS. 3A, 3B, there is shown an exemplary interleavingpattern used for interleaving frames of encoded bits in accordance withone embodiment of the present invention. FIG. 3 represents an exemplaryframe 61 of coded bits output from encoding block 14. In the exampleshown, the frame is 16 bits in length, the bits are numbered 0-15, andthe code used for coding the bits in frame 61 has rate R. Each frame ofcoded bits from block 14 is then applied to channel interleaver 18,which interleaves the bits in accordance with an interleaving patternthat facilitates early decoding of each received frame at a decodingsite as described above. An exemplary interleaving pattern thatfacilitates such early decoding is shown in connection with FIG. 3B,which illustrates an exemplary interleaved frame 62 output by block 18.In the example shown, the first fractional portion (a₁*100%) of theinterleaved frame corresponds to the first ¼ of the interleaved frame 62and includes bits coded at the code rate R1, where R1=R/a₁; the secondfractional portion (a₂*100%) of the interleaved frame corresponds to thefirst ½ of the interleaved frame 62 and includes bits coded at the coderate R2, where R2=R/a₂; and the third fractional portion (a₃*100%) ofthe interleaved frame corresponds to the first ¾ of the interleavedframe 62 and includes bits coded at the code rate R3, where R3=R/a₃.This interleaving pattern may be used in connection with an earlydecoding process that permits decoding attempts when ¼, ½, and ¾,respectively, of the frame is received by decoding unit 36. It will beunderstood by those skilled in the art that other values of a₁, a₂ anda₃ may be used for implementing the present invention, and that eachframe may be interleaved using only one or two or more than three of thefractional portions described above. It will also be understood by thoseskilled in the art that other interleaving patterns besides that shownin FIG. 3B may be used for implementing the early decoding process ofthe present invention, and that the use of such of interleaving patternsis within the scope of the present invention.

Referring now to FIG. 4, there is shown a further method for forming aninterleaved frame of coded bits, in accordance with an alternativeembodiment of the present invention. In the method shown in FIG. 4, eachframe of N information bits is applied to an encoder 66 that codes theinput bits using a code rate R. For each frame of N information bitsapplied to encoder 66, the encoder outputs a frame having N/R codedbits. Puncturing patterns p_(i) are then applied to each frame of codedinformation bits at block 74. A code of rate R/a_(i), is therebyproduced for each puncturing pattern p_(i). The values of a_(i),correspond to all of the possible fractions of the information frame atwhich detection is attempted within early decoding method 40. Forexample, a₁=¼, a₂=½, a₃=¾ can be used. This corresponds to attempting todecode when ¼, ½, and ¾, respectively, of the frame is received. Thisselection can be performed using brute force techniques. For example,the values ¼, ½, and ¾ can be tried and the results determined. Othervalues can then be tried and the results determined. Each puncturingpattern p_(i) is chosen so that the punctured coded bits contain thecoded bits obtained using puncturing pattern p_(i) if j<i. Thus, thebits produced by puncturing with the first pattern must be included inbits produced by subsequent (larger) patterns.

Interleaver 18 can then be selected in view of the puncturing patternsp_(i) as follows. Information bits 64 are applied to encoder 66.Information bits 64 have a length N. Encoder 66 has a rate R. Encodedbit stream 70 having a length N/R is thus provided at the output ofencoder 66. Puncturing device 74 receives encoded bit stream 70 andapplies a puncturing pattern pi to it. The output of puncturing device74 is applied to permutation function 78. Permutation function 78 can beany permutation function that provides N/R* a_(i) output bits 82 asshown within interleaver selection method 60.

Output bits 82 are inserted by interleaver 18 within first fraction 90of frame duration 86. First fraction 90 has a length a₁*100% the lengthof frame duration 86. If repeated attempts are made to decode the frame,N/R* a₂ output bits 82 and N/R* a₃ output bits 82 from permutationfunction 78 must be inserted into the appropriate fractions of frameduration 86 by interleaver 18. The inserted output bits 82 have lengthsof a₂*100 percent and a₃*100, respectively, of the length of frameduration 86 and have a starting time that coincides with frame duration86.

The previous description of the preferred embodiments is provided toenable a person skilled in the art to make and use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein can be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosed.

What is claimed is:
 1. A method for decoding a frame of interleavedinformation bits in a communications system, wherein the frame has aframe start time, a frame end time, and a first fractional segment thathas a start time that is the same as the frame start time and an endtime that is before the frame end time, comprising the steps of: (a)receiving interleaved information bits from the first fractional segmentof the frame; (b) attempting to decode the frame using information bitsfrom the first fractional segment of the frame; and (c) if the decodingattempt in step (b) is unsuccessful, attempting to decode the frameagain using information bits from both the first fractional segment ofthe frame and further information bits located between the end time ofthe first fractional segment and the frame end time.
 2. The method ofclaim 1, wherein the frame includes a second fractional segment that hasa start time that is the same as the start time of the first fractionalsegment and an end time that is the same as or before the frame endtime, and the further information bits are located in the secondfractional segment, the method further comprising: (d) if the decodingattempt in step (c) is unsuccessful, attempting to decode the frameagain using information bits from both the second fractional segment andadditional information bits located between the end time of the secondfractional segment and the frame end time.
 3. The method of claim 2,wherein the end time of the second fractional segment is the same as theframe end time.
 4. The method of claim 2, wherein the end time of thesecond fractional segment is before the frame end time, and the frameincludes a third fractional segment that has a start time that is thesame as the start time of the second fractional segment and an end timethat is the same as or before the frame end time, and the additionalinformation bits are located in the third fractional segment, the methodfurther comprising the step of: (e) if the decoding attempt in step (d)is unsuccessful, attempting to decode the frame again using informationbits from both the third fractional segment and still furtherinformation bits located between the end time of the third fractionalsegment and the frame end time.
 5. The method of claim 4, wherein theend time of the third fractional segment is the same as the frame endtime.
 6. The method of claim 4, wherein the end time of the thirdfractional segment is before the frame end time, further comprising thestep of: (f) if the decoding attempt in step (e) is unsuccessful,attempting to decode the frame again using all information bits from theframe.
 7. The method of claim 6, wherein the first fractional segment ofthe frame corresponds to a first quarter of the frame, the secondfractional segment of the frame corresponds to a first half of theframe, and the third fractional segment of the frame corresponds to afirst three-quarters of the frame.
 8. The method of claim 6, wherein a₁corresponds to a length of the first fractional segment of the frame, a₂corresponds to a length of the second fractional segment of the frame,a₃ corresponds to a length of the third fractional segment of the frame,the information bits between the frame start time and the frame end timeare encoded prior to step (a) at a code rate R, and the information bitsin the first, second and third fractional segments are encoded prior tostep (a) at code rates R1, R2 and R3, respectively, and R1=R/a₁,R2=R/a₂, and R3=R/a₃.
 9. The method of claim 1, wherein step (a) isperformed by a receiver and step (b) is performed before all of theframe has been received by the receiver.
 10. An apparatus for decoding aframe of interleaved information bits in a communications system,wherein the frame has a frame start time, a frame end time, and a firstfractional segment that has a start time that is the same as the framestart time and an end time that is before the frame end time,comprising: (a) a receiver that demodulates interleaved information bitsfrom the first fractional segment of the frame; and (b) a decoder thatattempts to decode the frame using information bits from only the firstfractional segment of the frame; wherein the decoder attempts to decodethe frame a further time using information bits from both the firstfractional segment of the frame and further information bits locatedbetween the end time of the first fractional segment and the frame endtime, if the decoder is unable to successfully decode the frame usinginformation bits from only the first fractional segment of the frame.11. An apparatus for decoding a frame of interleaved information bits ina communications system, wherein the frame has a frame start time, aframe end time, and a first fractional segment that has a start timethat is the same as the frame start time and an end time that is beforethe frame end time, comprising: (a) means for receiving interleavedinformation bits from the first fractional segment of the frame; (b)means for attempting to decode the frame using information bits fromonly the first fractional segment of the frame; and (c) means forattempting to decode the frame a further time using information bitsfrom both the first fractional segment of the frame and furtherinformation bits located between the end time of the first fractionalsegment and the frame end time if the attempt to decode the frame usinginformation bits from only the first fractional segment of the framefails.
 12. A method for improving the transmission of a frame ofinformation bits in a communications system, comprising the steps of:(a) receiving the frame of information bits within the communicationsystem; (b) encoding the information bits at a code rate R to provideencoded bits; (c) positioning the encoded bits in an interleaved framein accordance with an interleaving pattern that stores bits encoded at acode rate R1 at a first beginning fraction of the interleaved frame,wherein the starting time of the interleaved frame and starting time ofthe first beginning fraction coincide with each other; and (d)transmitting the interleaved frame.
 13. The method of claim 12, whereinR1=R/a₁, and a₁ corresponds to the first beginning fraction of theframe.
 14. The method of claim 12, wherein the length of the frame ofinformation bits is N, the number of bits in the first beginningfraction of the interleaved frame is N/R*a₁.
 15. The method of claim 12,wherein the interleaving pattern used in step (c) further stores bitshaving a code rate R2 at a second beginning fraction of the interleavedframe, wherein the starting time of the interleaved frame and startingtime of the second beginning fraction coincide with each other, and thesecond beginning fraction has an ending time that is after an endingtime of the first beginning fraction.
 16. The method of claim 15,wherein the interleaving pattern used in step (c) further stores bitshaving a code rate R3 at a third beginning fraction of the interleavedframe, wherein the starting time of the interleaved frame and startingtime of the third beginning fraction coincide with each other, and thethird beginning fraction has an ending time that is after an ending timeof the second beginning fraction.
 17. The method of claim 16, wherein a₂corresponds to a length of the second fractional segment of the frame,a₃ corresponds to a length of the third fractional segment of the frame,and interleaving pattern used in step (c) is chosen such that R2=R/a₂and R3=R/a₃.
 18. An apparatus for improving the transmission of a frameof information bits in a communications system, comprising: (a) anencoder that receives the frame of information bits and encodes theinformation bits at a code rate R to provide encoded bits; (b) aninterleaver that positions the encoded bits in an interleaved frame inaccordance with an interleaving pattern that stores bits encoded at acode rate R1 at a first beginning fraction of the interleaved frame,wherein the starting time of the interleaved frame and starting time ofthe first beginning fraction coincide with each other; and (c) atransmitter that transmits the interleaved frame through an informationchannel.
 19. An apparatus for improving the transmission of a frame ofinformation bits in a communications system, comprising: (a) means forreceiving the frame of information bits within the communication system;(b) means for encoding the information bits at a code rate R1 to provideencoded bits; (c) means for positioning the encoded bits in aninterleaved frame in accordance with an interleaving pattern that storesbits encoded at a code rate R1 at a first beginning fraction of theinterleaved frame, wherein the starting time of the interleaved frameand starting time of the first beginning fraction coincide with eachother; and (d) means for transmitting the interleaved frame through aninformation channel.